1. Field of the Invention
The invention relates to incoherent optical processing, particularly to a method and apparatus for performing Fourier transforms of image data using incoherent light.
This invention has significance in the area of automated image analysis for derivation of intelligence information from aerial photography or other image inputs. The Fourier Transform has proven itself to be useful for this purpose and this invention offers a lower noise Fourier Transform than is possible with known analog methods.
2. Discussion of the Related Art
The only known useful method for performing rapid parallel Fourier transforms of image data employs a coherent optical system. In such a system, coherent light, such as from a laser, is passed through a photographic transparency and focused to a point by a lens. The distribution of light around this focused point is an analog representation of the Fourier transform of the information contained in the photographic transparency. However, coherent noise can swamp the low-level information signal. This reduces the ability of the system to perform the subtle discrimination function which is essential to performing image analysis.
Incoherent Fourier transforms are not as easily constructed as coherent transforms. A series of sine and cosine masks can be multiplied by the input transparency and the resulting throughput integrated to give one point of the transform. In this manner, the transform may be built up point by point. This technique, however, has no advantage over digital methods in general use which are equally slow but have much lower noise.
To assure adequate speed in forming a Fourier transform, parallel processing of input information is required. An incoherent optical processing method using Fresnel zone plates is described in the literature. A Fresnel zone plate, commonly called simply a "zone plate", is a photographic transparency having a circularly symmetrical arrangement of alternating transparent and opaque zones whose boundaries are at radii proportional to the square roots of whole numbers. An input transparency located between zone plates suitably scaled and spaced will produce a Fourier transform in parallel at a properly chosen output plane.
Incoherent optical processing has an inherently lower noise level than coherent optical processing systems because of the channel redundancy associated with incoherent processing. Additionally, despite a lower throughput than equivalent coherent systems, the information throughput of an incoherent system is much higher than that of a serial processor such as an analog or digital electronic computer. However, a serious difficulty in incoherent processing methods is the presence of a large constant or bias signal, hundreds of times larger than the varying output signal sought. While the incoherent method should theoretically have lower noise, and therefore allow finer discrimination of input differences than coherent methods, the large bias term has prevented testing of this theory, and optical processing is currently performed exclusively by coherent optical systems despite their noisy character.